Occurrence and controls of preferential flow in the upper stream of the Heihe River Basin, Northwest China

Abstract

Preferential flow (PF) has a faster migration speed and is an important process affecting hydrological, geochemical and ecological cycles. However, the factors that control PF during infiltration are not fully understood, especially in the data-scarce high and cold mountainous areas, where few studies have been focused on PF. We selected eight soil moisture observation stations in the upper reaches of the Heihe River Basin (HRB), Northwest China. During the growing season from 2014 to 2019, the soil moisture was monitored every 30 min at 5 depths (5, 15, 25, 40, and 60 cm) at each station in order to analyze the occurrence and spatial–temporal control of PF in a cold, mountainous environment. According to the difference in response time of shallower and deeper soil moisture to rainfall, infiltration events were classified into PF and matrix flow. Our results show that the frequency of PF events across all layers of these stations ranged from 0 to 22.28%. As depth increased, the average PF frequency gradually decreased, but the coefficient of variation of PF frequency increased. The effect of vegetation on the occurrence of PF was profound, because vegetation affects root distribution and fauna activities, which affects the occurrence of PF. Our results also show that the frequency of PF gradually decreases from scrubland, to grassland and meadow, and barren land. Low bulk density, low residual water content, high saturated hydraulic conductivity, and high soil organic carbon were found to be conducive to the occurrence of PF. In addition, we found that PF possessed a high degree of seasonal variability because of its strong dependence on soil moisture, rainfall and vegetation conditions. High rainfall was conducive to PF occurrence. In areas with high sand content, low initial soil moisture was conducive to PF occurrence (likely due to hydrophobicity), whereas high initial soil moisture was conducive to PF occurrence in areas with low sand content. Overall, vegetation, soil properties, and water conditions seem to jointly affect the factors that occur in spatial–temporal of PF, and the influence of initial soil moisture on PF varies across different sand content. Findings of this study reveal the occurrence and control factors of PF, which will improve our understanding of water cycling related to the infiltration process, recharge process and the runoff generation process at hillslope over the cold mountainous areas.